An adaptive modulation and coding rate communication system has been proposed to present a high rate communication while sacrificing noise resistant characteristic to a user who holds a propagation path of a high quality, and to present a low rate communication while keeping the noise resistant characteristic to a user who only has a propagation path of a low quality, by changing the coding rate of an error correction code and the degree of multilevel modulation in accordance with the quality of the propagation path.
The adaptive modulation and coding rate communication system of this kind (here-in-after referred to as an adaptive and modulation system, accordingly) has recently been introduced in a radio communication system, and as for one example, there are GSM (Global System for Mobile Communications), EDGE, HDR (High Data Rate), 1xTREAM, and the like. Further, it is expected that the same communication system will be introduced into a communication situation using a W-CDMA (Wideband-Code Division Multiple Access).
In the situation using the adaptive coding and modulation system, the following fundamental procedures are carried out so as to attain the adaptive coding and modulation on the communication between a base station and a terminal.
1. The terminal determines the received quality of a signal which is transmitted from the base station.
2. The terminal notifies information representing the most suitable modulation system and coding rate (here-in-after referred to as a mode request message, accordingly) base on the determination result of the received signal quality to the base station.
3. The base station determines a modulation system and a coding rate to be actually assigned to the terminal based on a received signal quality message transmitted from the terminal and status of resources owned by the base station, and then transmits information representing the determined modulation system and the coding rate (here-in-after referred to as a transmission mode, accordingly) to the terminal.
4. The base station encodes and modulates the user data based on the coding rate and the modulation system corresponding to the transmission mode, and then transmits to the terminal.
5. The terminal receives the transmission mode and carries out a data receiving processing based on the received transmission mode.
6. If the terminal detects any error in the received data after carrying out the error correction, the terminal transmits a retransmission request, while if the terminal can accurately receive the data, the terminal transmits a new data transmission request back to the base station.
7. The above steps 1 to 6 are periodically repeated.
FIG. 1 illustrates a manner of the above mentioned processing sequence. As shown in this figure, illustrated are relations among a downlink control channel for notifying the transmission mode from the base station to the terminal, a downlink data channel for transmitting the user data from the base station, and an uplink control channel for transmitting a transmission mode request message from the terminal. An example shown in this figure is the one in which the above mentioned steps 1 to 6 are repeated at a predetermined frame period.
That is, as shown in FIG. 1, the terminal determines the received signal quality at the terminal at the current timing point, and then determines a transmission mode to request to the base station based on the received signal quality. Further, the terminal transmits a mode request message indicative of the transmission mode to the base station through the uplink control channel.
The base station determines a transmission mode based on the transmitted mode request message and radio resources owned by the base station, then transmits the thus determined transmission mode to the terminal through the downlink control channel. Furthermore, the base station carries out coding and modulation of the user data based on the coding rate and the modulation system corresponding to the transmission mode transmitted to the terminal, and then the transmits to the terminal through the downlink data channel.
Then the terminal receives the transmission mode transmitted from the base station in the previous stage, and the terminal recognizes the modulation system and the coding rate of the user data transmitted from the base station, thereby. The terminal further receives the user data transmitted from the base station thereafter and carries out the demodulation based on the demodulation system which corresponds to the modulation system designated by the transmission mode transmitted in advance.
The terminal carries out error detection of the user data which is obtained by the demodulation and the decoding, and, if no error is detected, then the terminal transmits a request message for new user data and a mode request message to the base station through the uplink control channel, for example.
On the other hand, if any error is detected in the user data obtained by carrying out the demodulation and the decoding, then the terminal transmits a retransmission request message requesting that the base station should retransmit the same user data through the uplink control channel to the base station. When the base station receives the retransmission request message, then the base station retransmits the user data again to the terminal.
The terminal receives the user data retransmitted from the base station, and carries out the error detection. And if the terminal detects any error in the user data, then the terminal transmits a retransmission request message again to the base station, and in this way, the similar processing is repeated. On the other hand, if the terminal detects no error in the user data after carrying out the error correction processing, as described above, the terminal transmits a request message for new user data and the mode request message to the base station through the uplink control channel, for example.
FIG. 1 contains terms of downlink data channel, downlink control channel, and uplink control channel. The words “downlink” and “uplink” mean a channel of a signal transmitted from the base station to the terminal and a channel of a signal transmitted from the terminal to the base station, respectively. That is, the word “downlink” is utilized for a name of a channel through which the signal is transmitted from the base station to the terminal and the word “uplink” is utilized for a name of a channel through which a signal is transmitted from the terminal to the base station.
Further, a retransmission request message can employ a flag of one bit, for example, and in this case, it is able to represent a request for retransmission if its value is “1”, and to represent a request for a new user data if it is “0”.
FIG. 2 is a diagram showing an example of an arrangement of a conventional base station which realizes a communication system employing an adaptive modulation and coding rate (adaptive coding and modulation system).
The base station is arranged to include a transmission/reception compatible unit 1, an inverse spreading unit 2, a power control bit extracting unit 3, a retransmission request message extracting unit 4, a mode request message extracting unit 5, a mode determining unit 6, a control unit 7, a control data generating unit 8, a coding and modulation unit 9, a power adjusting unit 10, a spreading unit 11, a retransmission buffer 12, an adaptive coding and modulation unit 13, and an antenna 14.
The base station demodulates the transmitted signal from a user by the transmission/reception compatible unit 1 and the inverse spreading unit 2.
That is, for example, a signal carried out a spectrum spreading is transmitted to the base station from a terminal capable of doing radio communication composed of a portable telephone, PDA (Personal Digital Assistant) or the like. The transmitted signal is received by the antenna 14 and supplied to the transmission/reception compatible unit 1. The transmission/reception compatible unit 1 is supplied with the transmission signal from the antenna 14 and subjects the signal to a necessary processing, and then supplies to the inverse spreading unit 2. The inverse spreading unit 2 carries out an inverse spectrum spreading on the signal supplied from the transmission/reception compatible unit 1 and supplies the resultant signal to the power control bit extracting unit 3.
The power control bit extracting unit 3 extracts a power control bit from the signal supplied from the inverse spreading unit 2. That is, the transmitted signal to be transmitted from the terminal to the base station contains the power control bit as a one-bit flag indicating a request of increase or decrease of power transmitted through the downlink control channel as described with reference to FIG. 1. The power control bit extracting unit 3 extracts this power control bit from the signal supplied from the inverse spreading unit 2 and transfers the power control bit to the power adjusting unit 10.
The power control bit extracting unit 3 extracts the power control bit from the signal supplied from the inverse spreading unit 2 and further supplies the signal to the retransmission request message extracting unit 4. The retransmission request message extracting unit 4 extracts the retransmission request message from the signal supplied from the power control bit extracting unit 3.
That is, the transmitted signal transmitted from the terminal to the base station contains the retransmission request message indicating whether the user data retransmission is requested or not, as described with reference to FIG. 1. The retransmission request message extracting unit 4 extracts the retransmission request message from the signal supplied from the power control bit extracting unit 3 and transmits the retransmission request message to the control unit 7.
The retransmission request message extracting unit 4 extracts the retransmission request message from the signal supplied from the power control bit extracting unit 3 and also transmits the signal to the mode request message extracting unit 5. The mode request message extracting unit 5 extracts the mode request message from the signal supplied from the retransmission request message extracting unit 4.
That is, the transmitted signal transmitted from the terminal to the base station contains the mode request message indicating the transmission mode determined based on the received signal quality at the terminal, as described with reference to FIG. 1. The mode request message extracting unit 5 extracts the mode request message from the signal supplied from the retransmission request message extracting unit 4 and then transfers the same to the mode determining unit 6.
In this case, the signal exchanged between the terminal and the base station is composed of frames each having a predetermined time span. Further, each frame is composed of a plural number, e.g., N, of slots of which time span unit is 0.6667 msec (millisecond), for example. The above described power control bit is arranged so that the power control bit is transmitted from the terminal to the base station at each slot. Therefore, the power control bit extracting unit 3 extracts the power control bit at every slot. Further, when the terminal transmits the signal, the retransmission request message and the mode request message are disposed at every frame. Therefore, the retransmission request message extracting unit 4 and the mode request message extracting unit 5 carry out respective extracting operations at every frame to extract the retransmission request message and the mode request message.
The mode determining unit 6 determines an optimum modulation system and coding rate in accordance with the mode request message and the state of the resources owned by the base station, and assigns code resources and power resources to the user (terminal).
Namely, if there is enough margin in the resources of the base station, the mode determining unit 6 determines the transmission mode represented by the mode request message as the transmission mode to be assigned as it is (here-in-after referred to as a assigned transmission mode, accordingly). On the contrary, if there is not any margin in the resources of the base station, the mode determining unit 6 determines the transmission mode which uses less resources as the assigned transmission mode instead of the transmission mode represented by the mode request message.
Then, when the mode determining unit 6 determines the assigned transmission mode, then transmits the assigned transmission mode to the control unit 7.
Initially, the control unit 7 confirms the retransmission request message transmitted from the retransmission request message extracting unit 4, and if it is confirmed that there is a retransmission request message, the control unit 7 transmits information indicative of a transmission mode identical to that upon initially transmitting the user data (i.e., the transmission mode upon transmitting the user data which is requested a retransmission) and a retransmission flag indicating that this transmission is a retransmission, to the control data generating unit 8 and the retransmission buffer 12. Further, if there is no retransmission request, the control unit 7 transfers a transmission mode (assigned transmission mode) determined by the mode determining unit 6 to the control data generating unit 8 and the adaptive coding and modulation unit 13.
The control data generating unit 8 generates control data including the transmission mode to be supplied from the control unit 7, and supplies to the coding and modulation unit 9. If the control data generating unit 8 is supplied with a retransmission request flag in addition to the information indicative of the transmission mode from the control unit 7, the control data generating unit 8 makes the retransmission request flag be involved in the control data. The coding and modulation unit 9 subjects the control data supplied from the control data generating unit 8 to a coding and modulation processing which is effected in a predetermined system, then, supplies the resultant modulated signal to the power adjusting unit 10.
The power adjusting unit 10 determines a level of transmission power for transmitting data through the downlink control channel described with reference to FIG. 1, in accordance with the power control bit supplied from the power control bit extracting unit 3. That is, as described above, the power control bit is a one-bit flag, for example. When the power control bit is “1”, then the power adjusting unit 10 increases the transmission power for the downlink control channel by 1 dB. When the power control bit is “0”, then the power adjusting unit 10 decreases the transmission power for the downlink control channel by 1 dB. Thus, the modulated signal supplied from the coding and modulation unit 9 is processed. In this way, it becomes possible to provide an arrangement for transmitting data through the downlink control channel to the terminal at an optimum power. Incidentally, in a communication using the CDMA, this manner of controlling the transmission power in the downlink control channel has been generally employed.
The modulated signal carried out the transmission power adjustment in the power adjusting unit 10 is supplied to the spreading unit 11.
Meanwhile, the packet data is supplied to the adaptive coding and modulation unit 13, wherein the user data is allocated in the packet data and the user data is to be transmitted through the downlink data channel which is described with reference to FIG. 1. The adaptive coding and modulation unit 13 subjects the packet data to a coding processing by using the coding rate which is indicated in the transmission mode information supplied from the control unit 7. Further, the adaptive coding and modulation unit 13 effects the modulation processing on the coded packed data by using the modulation system which is indicated in the transmission mode information. Thus, the modulated signal obtained by the coding and modulating the packet data is supplied to the retransmission buffer 12.
In this case, there are provided R=½ and R=¾ as coding rate (coding system) and QPSK (Quadrature Phase Shift Keying) and 16QAM (Quadrature Amplitude Modulation) as modulation system, three kinds of transmission modes #0 to #2 are explained here. In this case, the transmission mode is not limited to these three, and is able to prepare many types of modes by the combination of the coding rate and the modulation system.
The coding rate of R=½ means that a redundant bit of one bit is added to every one bit of input data. The coding rate of R=¾ means that a redundant bit of one bit is added to every three bits of input data. If the data is coded at the coding rate of R=½, as compared with a case in which the data is coded at the coding rate of R=¾, the whole data contains a larger number of redundant bits relative to the input data. Therefore, the error correction capability is strengthen but only small amount of data can be transmitted. Conversely, if the data is coded at the coding rate of R=¾, as compared with a case in which the data is coded at the coding rate of R=½, the whole data contains a smaller number of redundant bits relative to the input data. Therefore, the error correction capability is inferior to the case in which the data is coded at the coding rate of R=½ but a large amount of data can be transmitted.
Further, in the QPSK modulation system, as shown in FIG. 3A, the coded data are converted into symbols composed of two bits, and these symbols are mapped on any of four-symbol group, and in the 16QAM modulation system, as shown in FIG. 3B, the coded data are converted into symbols composed of four bits, and these symbols are mapped on any of 16-symbol group. If it is assumed that a symbol rate capable of being transmitted is constant, the modulation system of the 16QAM has a larger amount of data actually being transmitted than the modulation system of the QPSK. However, since the modulation system of the 16QAM has a smaller intersymbol distance than the modulation system of the QPSK, the noise characteristic is to be deteriorated.
Now that a combination of the coding rate of R=½ and the modulation system of the QPSK, a combination of the coding rate of R=½ and the modulation system of the 16QAM, and a combination of the coding rate of R=¾ and the modulation system of the 16QAM are defined as transmission modes of #0, #1, and #2, respectively, then the relation of the data transfer rate among these transmission modes is to be given as follows. That is, the transmission mode of #0 (R=½, QPSK)<the transmission mode of #1 (R=½, 16QAM)<the transmission mode of #2 (R=¾, 16QAM). On the other hand, the relation of the noise withstand characteristic among these transmission modes can be given as follows. That is, the transmission mode of #0 (R=½, QPSK)>the transmission mode of #1 (R=½, 16QAM)>the transmission mode of #2 (R=¾, 16QAM).
According to the adaptive coding and modulation system, the coding rate and the modulation system can be selectively determined depending on the nature of the signal transmission path. That is, if the noise level is low and the transmission path is good (i.e., the received signal quality at the terminal is good), a combination (transmission mode) of the coding rate and the modulation system providing a large amount of data transfer rate is selected to carry out effective data transmission. Conversely, if the noise level is high and the transmission path is bad (i.e., the received signal quality at the terminal is bad), a combination of the coding rate and the modulation system providing a high noise withstand characteristic (transmission mode) is selected to carry out data transmission in which the data transfer rate is suppressed and error correction characteristic is strengthened.
FIG. 4 is a diagram showing an arrangement of the adaptive coding and modulation unit 13 in which three transmission modes, i.e., modes of #0 to #2 shown in FIG. 2 are prepared.
The packet data inputted into the adaptive coding and modulation unit 13 is supplied to a switch 21.
If the transmission mode information supplied from the control unit 7 indicates the transmission mode #0, the switch 21 selects a terminal 21a and a switch 24 selects a terminal 24a. 
The terminal 21a is connected to a coding unit 22a. Therefore, if the transmission mode is of #0, the packet data is supplied from the switch 21 to the coding unit 22a. The coding unit 22a encodes the packet data supplied thereto at the coding rate of R=½ so that an error correction code is added to the data. The resultant coded data is supplied to a QPSK modulating unit 23a. The QPSK modulating unit 23a effects a QPSK modulation on the coded data supplied from the coding unit 22a so that modulated symbols are mapped to form a constellation. The resultant modulated signal is supplied to the terminal 24a of the switch 24. When the transmission mode is of #0, as described above, the switch 24 selects the terminal 24a. Therefore, the modulated signal outputted from the QPSK modulating unit 23a is supplied through the switch 24 to the retransmission buffer 12 (FIG. 2).
On the contrary, if the transmission mode information supplied from the control unit 7 indicates the transmission mode #1, the switch 21 selects a terminal 21b and the switch 24 selects a terminal 24b. The terminal 21b is connected to a coding unit 22b. Therefore, if the transmission mode is of #1, the packet data is supplied from the switch 21 to the coding unit 22b. The coding unit 22b encodes the packet data supplied thereto at the coding rate of R=½ and the resultant coded data is supplied to a 16QAM modulating unit 23b. The 16QAM modulating unit 23b effects a 16QAM modulation on the coded data supplied from the coding unit 22b and the resultant modulated signal is supplied to the terminal 24b of the switch 24. When the transmission mode is of #1, as described above, the switch 24 selects the terminal 24b. Therefore, the modulated signal outputted from the 16QAM modulating unit 23b is supplied through the switch 24 to the retransmission buffer 12 (FIG. 2).
Further, if the transmission mode information supplied from the control unit 7 indicates the transmission mode #2, the switch 21 selects a terminal 21c and the switch 24 selects a terminal 24c. The terminal 21c is connected to a coding unit 22c. Therefore, if the transmission mode is of #2, the packet data is supplied from the switch 21 to the coding unit 22c. The coding unit 22c encodes the packet data supplied thereto at the coding rate of R=¾ and the resultant coded data is supplied to a 16QAM modulating unit 23c. The 16QAM modulating unit 23c effects a 16QAM modulation on the coded data supplied from the coding unit 22c and the resultant modulated signal is supplied to the terminal 24c of the switch 24. When the transmission mode is of #2, as described above, the switch 24 selects the terminal 24c. Therefore, the modulated signal outputted from the 16QAM modulating unit 23c is supplied through the switch 24 to the retransmission buffer 12 (FIG. 2).
Now back to FIG. 2, the retransmission buffer 12 temporary stores the encoded and modulated user data supplied from the adaptive coding and modulation unit 13, and also supplies to the spreading unit 11. Further when the retransmission buffer 12 receives the retransmission flag from the control unit 7, then supplies the stored user data again to the spreading unit 11 as the retransmission data. In addition, when the retransmission buffer 12 does not receive the retransmission flag from the control unit 7, namely when the user data is normally received by the terminal, the retransmission buffer 12 erases (deletes) the normally received data in the stored user data.
The spreading unit 11 effects the spectrum spreading on the modulated signal supplied from the power adjusting unit 10 and the modulated signal supplied from the retransmission buffer 12 by using different spreading codes, and supplies thus obtained spread signal to the transmission/reception compatible unit 1. The transmission/reception compatible unit 1 effects a necessary processing on the spread signal supplied from the spreading unit 11 and transmits to the terminal as a radio wave from the antenna 14.
In this case, the modulated signal supplied from the power adjusting unit 10 is to be a signal transmitted through the downlink control channel shown in FIG. 1 and the modulated signal supplied from the adaptive coding and modulation unit 13 is to be a signal transmitted through the downlink data channel shown in FIG. 1.
Next, FIG. 5 is a diagram showing an example of an arrangement of a conventional terminal which can realize a communication system employing an adaptive modulation and coding rate (adaptive coding and modulation system).
The terminal (user terminal) is arranged to include a transmission/reception compatible unit 31, an inverse spreading unit 32, a control channel received signal quality estimating unit 33, a power control bit generating unit 34, a data channel received signal quality estimating unit 35, a mode selection unit 36, a control data demodulating and decoding unit 37, a control unit 38, a user data demodulating and decoding unit 39, an error detecting unit 40, a retransmission request message generating unit 41, a retransmission request message inserting unit 42, a mode request message inserting unit 43, a power control bit inserting unit 44, a spreading unit 45, and an antenna 46.
A transmitted signal transmitted from the base station is received by the antenna 46. The received signal is subjected to the necessary processing in the transmission/reception compatible unit 31, and thereafter supplied to the inverse spreading unit 32. The inverse spreading unit 32 effects an inverse spectrum spreading on the signal supplied from the transmission/reception compatible unit 31 so that the signal is divided into a signal for the downlink data channel and a signal for the downlink control channel which are described with reference to FIG. 1. The inverse spreading unit 32 supplies the signal for the downlink control channel to the control channel received signal quality estimating unit 33 and the control data demodulating and decoding unit 37. Further, the inverse spreading unit 32 supplies the signal for the downlink data channel to the data channel received signal quality estimating unit 35 and the user data demodulating and decoding unit 39.
The control channel received signal quality estimating unit 33 estimates a signal to noise ratio (SNR (Signal to Noise Ratio)) based on a pilot signal which derives from time division multiplexing effected in the downlink control channel. That is, although description is not provided with reference to FIG. 2, the spreading unit 11 carries out a time division multiplexing on a predetermined pilot signal with the demodulated signal supplied from the power adjusting unit 10. Thereafter, the spreading unit 11 carries out the spectrum spreading on the signal. Therefore, the signal transmitted through the downlink control channel contains the pilot signal in addition to the modulated signal supplied from the power adjusting unit 10. The control channel received signal quality estimating unit 33 estimates the SNR of the signal supplied from the inverse spreading unit 32 through the downlink control channel by using the pilot signal contained in the signal. Then, the control channel received signal quality estimating unit supplies the estimated SNR to the power control bit generating unit 34.
The power control bit generating unit 34 responds to the estimated SNR of the downlink control channel in such a manner that if the estimated SNR is better than a desired SNR then a power control bit of a value “0” is outputted to the power control bit inserting unit 44 while if the same is worse than the desired value then a power control bit of a value “1” is outputted to the power control bit inserting unit 44. In this case, the SNR estimation in the control channel received signal estimating unit 33 and the power control bit generation in the power control bit generating unit 34 are executed for every slot. Then, the base station described with reference to FIG. 2 controls the transmission power of the downlink control channel based on the power control bit so that the terminal can always receive the signal of the downlink control channel at a constant SNR.
The control data demodulating and decoding unit 37 demodulates and decodes the signal supplied from the inverse spreading unit 32 through the downlink control channel, separates the control data from the signal, and supplies the same to the control unit 38.
The control unit 38 detects the information concerning the coding rate and the modulation system to be applied to the downlink data channel, i.e., the transmission mode information, which is disposed in the control data supplied from the control data demodulating and decoding unit 37. Then, the control unit carries out mode setting (control) for the user data demodulating and decoding unit 39.
That is, as shown in a flowchart of FIG. 6, initially at step S1, the control unit 38 detects the transmission mode from the control data supplied from the control data demodulating and decoding unit 37 and the processing proceeds to step S2. At step S2, the control unit 38 examines whether the modulation system indicated by the transmission mode is the QPSK modulation or not. At step S2, if it is determined that the modulation system indicated by the transmission mode is the QPSK modulation, the processing proceeds to step S3. In this step S3, the control unit 38 demodulates the signal of the downlink data channel based on the QPSK manner, and controls the user data demodulating and decoding unit 39 so that this unit decodes the signal at the coding rate of R=½. Thereafter, the control unit 38 awaits the next control data to be supplied from the control data demodulating and decoding unit 37, and then, the processing returns from step S3 to S1, and the same processing sequence is repeated in a similar manner.
At step S2, if it is determined that the modulation system indicated by the transmission mode is not the QPSK modulation, the processing proceeds to step S4. In this step S4, the control unit 38 examines whether the modulation system indicated by the transmission mode is the 16QAM and the coding rate indicated by the transmission mode is R=½ or not. At step S4, if it is determined that the modulation system indicated by the transmission mode is the 16QAM and the coding rate indicated by the transmission mode is R=½, then the processing proceeds to step S5. In this step S5, the control unit 38 demodulates the signal of the downlink data channel based on the 16QAM manner, and controls the user data demodulating and decoding unit 39 so that this unit decodes the signal at the coding rate of R=½. Thereafter, the control unit 38 awaits the next control data to be supplied from the control data demodulating and decoding unit 37. The processing returns from step S5 to S1, and the same processing sequence is repeated in a similar manner.
At step S4, if it is determined that the transmission mode information does not designate the combination of the modulation system of the 16QAM and the coding rate of R=½, the processing proceeds to step S6. In this step S6, the control unit 38 examines whether the modulation system indicated by the transmission mode is the 16QAM or not and whether the coding rate indicated by the transmission mode is R=¾ or not. At step S6, if it is determined that the modulation system indicated by the transmission mode is the 16QAM and the coding rate indicated by the transmission mode is R=¾, then the processing proceeds to step S7. In this step, the control unit 38 demodulates the signal of the downlink data channel based on the 16QAM manner, and controls the user data demodulating and decoding unit 39 so that this unit decodes the signal at the coding rate of R=¾. Thereafter, the control unit 38 awaits the next control data to be supplied from the control data demodulating and decoding unit 37, and then, the processing returns from step S7 to S1, and the same processing sequence is repeated in a similar manner.
At step S6, if it is determined that the transmission mode information does not designate the combination of the modulation system of the 16QAM and the coding rate of R=¾, this determination means that the transmission mode information does not designate the above mentioned three combinations of the modulation system and the coding rate. In this event, the control unit 38 determines that the transmission mode is erroneous one, and hence the control unit takes no particular action in controlling the user data demodulating and decoding unit 39. Thus, the control unit 38 awaits the next control data to be supplied from the control data demodulating and decoding unit 37, and the processing returns from step S6 to S1, and the same processing sequence is repeated in a similar manner.
Now description will be again made with reference to FIG. 5. The data channel received signal quality estimating unit 35 estimates the SNR of the signal of the downlink data channel supplied from the inverse spreading unit 32. When the data channel received signal quality estimating unit 35 estimates the SNR, the data channel received signal quality estimating unit 35 utilizes a pilot symbol subjected to the time division multiplexing on the downlink data channel or a pilot channel symbol transmitted together with the downlink data channel in a parallel manner.
Although description is not provided yet with reference to FIG. 2, the spreading unit 11 effects the time division multiplexing on the predetermined pilot signal with the demodulated signal supplied from the adaptive coding and modulation unit 13. Thereafter, the spreading unit 11 carries out the spectrum spreading. Therefore, the signal of the downlink data channel contains the pilot signal. Further, the spreading unit 11 effects the spectrum spreading on another pilot signal with a spreading code different from a spreading code which is utilized for the effecting spectrum spreading on the demodulated signal supplied from the power adjusting unit 10 or the adaptive coding and modulation unit 13. Then, the pilot signal is supplied through the transmission/reception compatible unit 1 to the antenna 14 from which the pilot signal is transmitted through the downlink data channel and the downlink control channel in parallel.
The data channel received signal quality estimating unit 35 estimates the SNR of the signal of the downlink data channel supplied from the inverse spreading unit 32 by using the pilot signal contained in the signal or the pilot signal transmitted in parallel together with the signal of the downlink data channel. Then, the estimated SNR is supplied to the mode selection unit 36.
The mode selection unit 36 recognizes the estimated SNR of the downlink data channel supplied from the data channel received signal quality estimating unit 35 as the received signal quality at the terminal, and determines (selects) the combination, namely the transmission mode, of the coding rate and modulation system capable of setting the error rate of the user data to be equal or less than the predetermined value relative to the received signal quality. Further, the mode selection unit 36 generates a mode request message indicating its transmission mode as to request thus determined transmission mode to the base station, and supplies to the mode request message inserting unit 43.
In this case, the data channel received signal quality estimating unit 35 estimates the SNR of the downlink data channel for each frame, and the mode selection unit 36 generates the mode request message also at every frame.
In this case, if it is estimated that the noise level of the transmission path is small, accordingly, the transmission path is better (when the received signal quality is better) based on the received signal quality supplied from the data channel received signal quality estimating unit 35, the mode selection unit 36 selects the transmission mode combining the coding rate and the modulation system which are able to handle higher transmission rate data. Conversely, if it is estimated that the noise level of the transmission path is high and, accordingly the transmission path is bad (when the received signal quality is extremely bad), the transmission mode combining the coding rate and the modulation system which are able to suppress the rate of the data transmission and to strengthen the error correction.
More specifically, the mode selection unit 36, as shown in FIG. 7, for example, selects the transmission mode capable of setting the error rate of the user data to be equal or less than the predetermined value.
Namely, FIG. 7 shows a relation between the received signal quality and the error rate of the user data (FER: Frame Error Rate) with respect to above mentioned respective three transmission modes #0 (R=½, QPSK), #1 (R=½, 16QAM), and #2 (R=¾, 16QAM). The mode selection unit 36, for example, selects the transmission mode capable of setting the error rate (FER) of the user data to be equal or less than 10% relative to the received signal quality. In this case, according to FIG. 7, the mode selection unit 36 selects the transmission modes #0 (R=½, QPSK), #1 (R=½, 16QAM), or #2 (R=¾, 16QAM), respectively in case when the received signal quality is below −8 db, larger than −8 db and smaller than −4 db, or more than −4 db.
On the other hand, the user data demodulating and decoding unit 39 carries out decoding and demodulation on the signal of the downlink data channel supplied from the inverse spreading unit 32 under the control of the control unit 38 which is described with reference to FIG. 6. The resultant user data obtained by the operation is supplied to the error detecting unit 40. When the user data demodulating and decoding unit 39 decodes the signal of the downlink data channel, the user data demodulating and decoding unit 39 carries out user data error correction by using the error correction code contained in the signal as a redundancy bit.
The error detecting unit 40 carries out a parity detection by using Cyclic Redundancy Check (CRC), for example. That is, the error detecting unit 40 examines whether the user data decoded by the user data demodulating and decoding unit 39 contains error or not, and the error detecting unit 40 supplies the result of the examination to the retransmission request message generating unit 41.
If the retransmission request message generating unit 41 receives an examination result indicating that there is no error contained therein from the error detecting unit 40, then the retransmission request message generating unit 41 generates a message having a value of “0”, for example, and supplies the message to the retransmission request message inserting unit 42. Conversely, if the retransmission request message generating unit 41 receives an examination result indicating that there is some error contained therein from the error detecting unit 40, then the retransmission request message generating unit 41 generates a message having a value of “1”, for example, and supplies the message to the retransmission request message inserting unit 42.
The retransmission request message inserting unit 42 carries out framing on the retransmission request message supplied from the retransmission request message generating unit 41 with the signal of the uplink control channel described with reference to FIG. 1, and supplies the resultant signal to the mode request message inserting unit 43. The mode request message inserting unit 43 carries out framing on the mode request message supplied from the mode selection unit 36 with the signal of the uplink control channel supplied from the retransmission request message inserting unit 42. The resultant signal is supplied to the power control bit inserting unit 44. The power control bit inserting unit 44 carries out framing on the power control bit supplied from the power control bit generating unit 34 with the signal of the uplink control channel supplied from the mode request message inserting unit 43. The resultant signal is supplied to the spreading unit 45. The spreading unit 45 effects the spectrum spreading on the signal of the uplink control channel transmitted from the power control bit inserting unit 44, and supplies the resultant signal obtained by the operation to the transmission/reception compatible unit 31. The transmission/reception compatible unit 31 effects a necessary processing on the spread signal transmitted from the spreading unit 45 and transmits the signal through the antenna 47.
As described above, according to the adaptive coding and modulation system, the terminal informs (requests) a desirable combination (transmission mode) of the modulation system and coding rate to the base station based on the received signal quality, then the base station is able to select the combination of the modulation system and the coding system to be actually assigned based on this, so that it becomes possible to change the data transmission speed based on the receiving condition (received signal quality) at the terminal, and accordingly the data can be transmitted to the terminal more effectively.
However, it is impossible for the terminal to determine when the user data is transmitted, so that it is necessary to always transmit a mode request message to the base station. The uplink wireless resources are employed when the mode request message is transmitted to the base station, so that interferences become huge when there are a lot of terminals. Further, in the base station, the transmission of mode request massage from the terminal to the base station, although there is no data to be transmitted to the terminal, causes useless power consumption in the terminal.